PROJECT SUMMARY Despite strong consensus that early, specialized intervention for children with Autism Spectrum Disorder (ASD) can have a dramatic impact on outcomes, the U.S. health system?s capacity to provide such services is severely challenged by the rapid rise in ASD prevalence. The long-term goal is to improve outcomes for children with early signs of ASD by increasing the capacity to provide appropriately specialized treatment within an existing infrastructure: the Part C Early Intervention (EI) service delivery system. Part C is publicly funded, available in all areas, and serves children under age 3 who have developmental delays or disabilities. Currently, the effectiveness of EI services is limited by high practice variation and infrequent use of evidence- based interventions. The study objective is to improve services and outcomes for children with early signs of ASD by conducting a randomized controlled trial (RCT) testing the effectiveness of training EI providers to deliver Reciprocal Imitation Training (RIT). RIT is a naturalistic developmental behavioral intervention (NDBI) that is ideally suited for EI settings because it is low intensity, play-based, easy to learn and implement, and can be taught to families for their independent use, thus increasing intervention dosage. This RCT will employ a hybrid type 1 effectiveness/implementation design, and will use a unique mixed methods approach to gather evidence that will be essential for implementing RIT at scale, pending positive trial results. The sample includes 20 EI agencies across 4 U.S. States and comprises a total of 160 EI providers and 440 families of children with early symptoms of ASD, which not only provides a robust sample size, but also affords the opportunity to assess generalizability of this approach across regions that vary in their implementation of Part C services. EI providers will be randomly assigned to the RIT training group (n=80) or treatment as usual (TAU; n=80). Providers in both groups will identify 2-5 children in their caseload who are 16-30 months old with early symptoms of ASD (n=220 children per group). Intensive, state-of-the art, multi- method assessment technology will be used to measure the impact of the intervention on children?s language and social communication, as well as parents? self-efficacy and well-being. Importantly, this study will examine putative mechanisms (i.e., child gains in imitation and joint attention; parent contingent responsiveness) through which the intervention improves clinically-relevant outcomes. Data regarding provider-initiated modifications to the intervention and delivery will be analyzed to identify fidelity-consistent vs. fidelity- inconsistent changes, which will inform refinement of future RIT training and quality assurance procedures. In sum, this study will generate the evidence necessary to implement RIT at scale, thereby increasing the capacity of the existing EI system to deliver effective, evidence-based intervention to the rapidly growing population of children who show early signs of ASD.